We wish to investigate the changes which occur in the enzymatic specificity of DNA polymerases and RNA polymerases isolated from human fibroblasts which are aging in culture and to understand the role of these changes in the senescence and death of the cell. Our work is a direct outgrowth of our previous observation that the DNA polymerase activity from late passage cells copies DNA synthetic templates less faithfully than the polymerase from early passage cells. We will perform experiments to determine the biochemical basis of this infidility in DNA polymerase and, by using in vitro mis-incorporation assays, we will examine the accuracy of RNA polymerases obtained from fibroblasts which have a defined life span in culture. We will compare the in vitro error frequencies of DNA and RNA polymerases from young and old cells to those of similar enzymes extracted from cells of patients with progeriod syndromes, from transformed cells with infinite proliferative capacity, and from tissues of young and old animals. It is hoped that an understanding of the accuracy of these nuclear functions on the biochemical level can be related to cellular deterioration and the cessation of replication in the case of cells with limited life span and to degenerative changes in tissues of animals as they age.